[Walt Fricke]

Steve - you don't say that you have replaced the rubber brake lines. While this has not happened to me, it is pretty standard for someone who takes his car on the track or autocrosses it to replace the rubber ones with flourocarbon steel braided lines. Or with new rubber lines.

The common wisdom is that as the rubber degrades with time it is possible for a flap to form, and the flap may form in such a way as to create a one way valve - the one way being fluid to the caliper, thus blocking the return flow when the master cylinder returns to its "off" position, thus opening up the circuit to atmospheric pressure and allowing any pressure to bleed back to the MC, and through it if needed to the reservoir.

Sophos did this replacement, which reduces the number of usual suspects.

I see you are planning to fiddling with the linkages. That must might bear fruit, because if the linkage is "too long," it might prevent the MC from retracting fully. Which will do precisely what you are experiencing. In Sophos' case, he pulled out one component of the linkage, and it did not relieve the pressure. But you may be more fortunate.